1. Field of the Invention
The present invention relates to a voltage-controlled oscillator (VCO), which oscillates at a frequency proportional to a control input voltage, and more particularly to an LC-tank voltage-controlled oscillator (LC-VCO).
2. Description of the Related Art
The phase noise of an LC-tank voltage-controlled oscillator is determined by the S/N ratio (Signal-to-Noise Ratio) in the LC tank. The root mean square values of signal voltage Vsig and noise voltage Vn are respectively expressed asVsig2=Rt2Isig2 Vn2=4kTRt where k is the Boltzmann constant, T is the absolute temperature, Rt is the parallel impedance of the LC tank, and Isig is the signal current amplitude.
Hence, the root mean square value of the S/N ratio is given as(S/N)2=Rt2Isig2/4kT From the above equation, it can be seen that phase noise improves with increasing signal amplitude. However, since the signal amplitude is limited by supply voltage, the phase noise degrades when VCO is operated at a low supply voltage.
In the prior art, a transformer-feedback (TF) VCO is proposed as a VCO that operates at a low supply voltage (refer to non-patent document 1 listed below). In this transformer-feedback VCO, each source node voltage in a cross-coupled transistor pair is lowered below 0 [V] by a transformer, and the voltage amplitude in the LC tank is increased. However, since the operation involves current flow in a region where the impulse sensitivity function (ISF) is large, it is difficult to achieve a low-phase-noise design with this technique.
On the other hand, an LC-VCO known as a Class-C VCO is proposed (refer to non-patent documents 2 and 3 listed below). This VCO is constructed so that, in an LC-VCO having a cross-coupled transistor pair, the transistor gate portion is DC-cut by a capacitor to lower the gate bias below the threshold, thereby causing the transistor to operate as if it were a class-C amplifier.
In a VCO, the ISF from thermal noise to phase noise differs depending on the phase of oscillation. More specifically, the ISF is smaller for a phase where the amplitude of the oscillation voltage is larger. The class-C VCO is intended to achieve low phase noise by flowing a current only in a phase range where the ISF is small. However, in the class-C VCO, the gate bias must be reduced below the threshold voltage. As a result, under low supply voltage conditions, the amplitude cannot be made large enough, and oscillations may not occur. On the other hand, under low amplitude conditions, the conduction angle cannot be reduced to take advantage of the characteristics of the class-C VCO, and the intended low phase noise cannot be achieved.
Non-patent document 1: K. Kwok, and H. C. Luong, “Ultra-Low-Voltage High-Performance CMOS VCOs Using Transformer Feedback,” IEEE JSSC, vol. 40, no. 3, pp. 652-660, March 2005.
Non-patent document 2: A. Mazzanti, and P. Andreani, “Class-C Harmonic CMOS VCOs, With a General Result on Phase Noise,” IEEE JSSC, vol. 44, no. 12, pp. 2716-2729, December 2008.
Non-patent document 3: A. Mazzanti, and P. Andreani, “A 1.4 mW 4.90-to-5.65 GHz Class-C CMOS VCO with an Average FoM of 194.5 dBc/Hz,” ISSCC, No. 26-2, February 2008.